In the field of scanning probe microscopy, small probes interact with samples under test to measure mechanical properties of the samples under test. For example, in atomic force microscopy, a small probe (typically sub-micrometer sized) is attached to the end of a cantilever. As the probe is scanned across the surface of a sample under test, such as the membrane of a cell, surface irregularities impose a varying force on the probe, which, in turn, results in a bending or deflection of the cantilever. An optical sensor senses the deflection of the cantilever based on light reflected from the cantilever and thereby determines changes in normal position of the probe as it is scanned across the surface of the sample under test. The changes in normal position of the probe are used to map the surface of the sample under test.
FIG. 1A illustrates a typical application of atomic force microscopy. In FIG. 1A, a probe 100 is attached to the end of cantilever 102 to map the surface 104 of a cell membrane. A laser, an optical sensor, and a computer (not shown) are used to map surface 104 as probe 100 causes deflections in cantilever 102. One problem with atomic force microscopy is illustrated in FIG. 1B. Atomic force microscopy requires a mechanical connection between probe 100 and the remainder of the system via cantilever 102. As a result, conventional atomic force microscopy is unsuitable for measuring mechanical properties of structures within enclosed regions, such as organelles within a cell membrane, or the other structures that are inaccessible for scanning with a mechanically attached probe.
One way to measure properties of structures inside of cells and other enclosed environments is to mechanically decouple the probe from the remainder of the system. However, once the probe is mechanically decoupled from the remainder of the system, tracking and controlling movement of the probe become problematic. One known technique of applying force to a mechanically decoupled probe is referred to as “optical tweezers.” This technique requires high optical field intensities that interact strongly with many materials and may produce undesirable side effects on experiments in biological samples.
Commonly-assigned, co-pending international patent application number PCT/US02/30853 describes a magnetic coil and pole assembly with four pencil-shaped pole pieces that converge from the vertices of an equilateral tetrahedron. Although such an assembly is useful in many environments, it may be desirable to control the motion of a probe in microscopes having high numerical aperture (NA) objective lenses with short focal distances. For example, some lenses may have numerical apertures greater than or equal to one at focal distances on the order of millimeters. Such lenses typically have large diameters and thus limit the space for placement of magnetic pole pieces used to control the motion of mechanically unattached probes. The space for placing pole pieces is even further limited when high NA objective lenses are placed both above and below the sample under test. In addition, at some positions within the volume defined by the pole pieces in a four-pole system, moving the probe in certain directions can be difficult.
Another factor to be considered in designing and placing magnetic pole pieces to control motion of a mechanically unattached magnetic probe is that the magnetic force on the probe for a given magnetic field varies inversely with r5, where r is the distance from the pole tip applying the magnetic force to the magnetic probe. Thus, in order to apply strong magnetic forces to a probe, it is desirable that the pole tips be kept as close as possible to the probe. However, because the pole tips compete for space with imaging and tracking optics, designing a system that achieves desired magnetic forces and that is compatible with high-resolution optics is difficult.
Accordingly, there exists a long felt need for improved magnetic structures for applying magnetic force to a mechanically unattached probe that are suitable for use with high resolution optics or in other space-constrained environments.